Every microphone system has a directivity pattern indicative of its response based on the location of a sound source. Directivity patterns include, for example, cardioid and hypercardioid. Microphones can be customized to feature omnidirectional, bidirectional and unidirectional directivity. However, microphones featuring a rear lobe of directivity effectively reduces microphone efficiency and directional performance.
FIG. 1 illustrates the directivity patterns for a prior art unidirectional microphone. The results can be obtained using omnidirectional microphone elements and a bidirectional element, or by using a modified bidirectional microphone. Each result, however, results in a Directivity Index (“DI”) of less than 6 decibels (dB). As used throughout this disclosure, the term DI refers to a measurement of the resistance to diffuse noise by a microphone element. The greater the DI, the greater the microphone element resists diffuse noise, i.e., the less diffuse noise is ‘picked up’ or received by the microphone element. Another effect of increasing the DI for a microphone is a resulting increase in the acceptable distance between microphone and sound source while maintaining a constant signal level.
For example, a current microphone in use has a DI of 5 dB. This microphone works best within about 16 inches of the sound source. Increasing the DI to 9 dB would increase the microphone range to about 22.5 inches.
Prior solutions to increase the DI of microphones have required use of either expensive equipment, such as parabolic arrays, or sizable equipment inappropriate for use in space-limited applications such as a mobile vehicle.